1. Field of the Invention
The present invention relates to a Mn—Zn based ferrite member to be suitably used as a core in a coil and as a core in a transformer, in particular, a Mn—Zn based ferrite member to be suitably used as a core for a small low-profile power coil and as a core for a small low-profile power transformer each displaying a low loss, a high withstand electric voltage (hereinafter referred to as withstand voltage) and excellent magnetic properties, and having high mass productivity.
2. Description of the Related Art
As the cores for small low-profile power devices, there have hitherto been used cores formed by using Ni—Zn ferrite, directly winding wire therearound and forming terminal electrodes, or cores formed by combining an insulating bobbin and Mn—Zn based ferrite. The former type permits downsizing but is large in loss and poor in magnetic properties, and has a problem such that the direct current superposition property is insufficient. On the other hand, the latter type is small in loss, excellent in magnetic properties, and permits, for example, forming a coil having an excellent direct current superposition property; however, this type has a small core resistance, which necessitates combinational use of an insulating bobbin, and thereby makes downsizing of this type difficult.
In view of the above described circumstances, various proposals have been made for cores excellent in magnetic properties, low in loss and permitting downsizing.
Among such proposals is Japanese Patent Publication No. 2-60073 (Patent Document 1) which discloses a technique to form a high electric resistance layer on the surface by oxidizing a Mn—Zn based ferrite having a particular composition in a high temperature oxidative atmosphere. However, the material composition of the Mn—Zn based ferrite in Patent Document 1 is such that ZnO: 14.6 to 29.8 mol % (10 to 20 wt %) and MnO: 17.1 to 33.4 mol % (10 to 20 wt %), namely, a high μ composition; the saturation magnetic flux density (Bs) and the direct current superposition property of the ferrite are poor.
Japanese Patent Laid-Open No. 6-295812 (Patent Document 2) also discloses a technique in which the surface of a Mn—Zn based ferrite (Fe2O3: 50 to 56 mol %, ZnO: 5 to 25 mol %, the balance being MnO (19 to 45 mol %)) is oxidized to form a high resistance layer. However, the Mn—Zn based ferrite member obtained in Patent Document 2 has a small surface layer resistance of the order of 1×103 Ωm, accordingly the withstand voltage being hardly made sufficient. In Patent Document 2, the oxidation treatment is carried out by introducing air in a temperature range between 700 and 400° C. in the cooling step after sintering. The cause for the insufficient development of the surface resistance may be understood that the introduction of air decreases the temperature of the atmosphere in the course of the oxidation treatment, thus preventing the oxidation from proceeding sufficiently.
Japanese Patent Laid-Open No. 5-275225 (Patent Document 3) also discloses a technique to form a high resistance layer by oxidizing the surface of a Mn—Zn based ferrite. In Patent Document 3, a press compacted body composed of the Mn—Zn based ferrite powder is sintered, and then the sintered body is subjected to the surface oxidation treatment in the air atmosphere at 600 to 800° C. In the Mn—Zn based ferrite core obtained in Patent Document 3, the oxidation proceeds to the interior of the sintered body in such a way that the resistance value at the depth of 50 μm from the surface is 10 or more times the resistance value at the depth of 200 μm from the surface; thus the withstand voltage is high, but the loss comes to increase drastically.
Japanese Patent No. 3108803 (Patent Document 4) discloses a method in which the volume resistivity of the whole core is increased by regulating additives in such a way that the Mn—Zn based ferrite composition has an iron-poor composition. Although this method can give a sufficient withstand voltage, the iron-poor composition drastically increases the loss, and moreover, degrades the magnetic properties so as to be equivalent to those of Ni—Zn based ferrite.
Japanese Patent Laid-Open No. 6-283357 (Patent Document 5) discloses a technique to form an inorganic high resistance coating film on the surface of a core, while Japanese Patent Laid-Open No. 5-335156 (Patent Document 6) discloses a technique to form an organic high resistance coating film on the surface of a core. These techniques each can decrease the core loss and can provide a high withstand voltage and high magnetic properties, but the step to form the coating film is cumbersome and these techniques are thereby poor in mass productivity.
[Patent Document 1] Japanese Patent Publication No. 2-60073.
[Patent Document 2] Japanese Patent Laid-Open No. 6-295812.
[Patent Document 3] Japanese Patent Laid-Open No. 5-275225.
[Patent Document 4] Japanese Patent No. 3108803.
[Patent Document 5] Japanese Patent Laid-Open No. 6-283357.
[Patent Document 6] Japanese Patent Laid-Open No. 5-335156.